As far as I am aware and I know that many people especially from Pakistan pronounce the letter "thaa" as "saa" due to the Urdu alphabet, there is no sin if someone mispronounces, unintentionally, a verse from the Qur'an because due to their accent and the style of their mother tongue. They should try to correct it because it is not ideal, but if they are unable to because their accent is quite strong, they are excused. But this is the exception, not the rule. People should try their best to master the Arabic pronunciation. If you decide not to correct your pronunciation, and you have been advised to correct it but do not heed this advice, then it is sinning due to ignorance and arrogance.
“And We have indeed made the Qur’aan easy to understand and remember; then is there any one who will remember (or receive admonition)?”[al-Qamar 54:17]